The Influence Of FWM Effect On CO-OFDM System Performance

CO-OFDM technology combining the advantages of OFDM and coherent optical communication technology, which is considered as one of the most important modulation technology in optical communication, has high spectral efficiency and receiver sensitivity. In fact there are a few physical layer impairments including FWM effect as well in optical communication which severely damage system performance.Therefore, It is of great importance to study the high speed and long distance CO-OFDM system and analyze transmission performance.Firstly, related fundamental theories and systemic concept are illustrated in CO-OFDM system. Then the performance, implementation and some key technologies of CO-OFDM were deeply investigated by constructing the CO-OFDM model. Some major research contents and achievements are summarized as follow:(1) CO-OFDM system model is established in this paper which is used to study the implementation of CO-OFDM. From the formula derivation of basic theories based on the study of basic principles and key technologies about OFDM technology,the whole process of transmission signals being modulated, transported and detected reach the end of receiver of CO-OFDM system. Also combined with the current status of domestic and foreign research, advantages, disadvantages and some existedproblems of CO-OFDM are studied in details.(2) Two methods about modulations, QAM modulation and QPSK modulation,are discussed in CO-OFDM system. The optical communication modulated by16 QAM and QPSK separately whose bit rate is 30375000 bits/s, and sample rate is1.944 GHz, is studied by Optisystem software. The simulations results show that two modulations both can improve systemic bandwidth efficiency and resist interference,also QPSK modulation in CO-OFDM system has lower bandwidth efficiency compared to 16 QAM modulation, it has stronger power efficiency and lower Bit Error Rate(BER) than QPSK modulation.(3) Physical layer impairment(PLI) model where amplified spontaneous emission(ASE) categorized to linear impairments and four-wave mixing(FWM) categorized to nonlinear impairments are regarded as the main noise sources, based on Q factor is established in this paper, which is used to study that FWM effect how to influence performance of CO-OFDM system. Compared to existed PLI model, the new model is able to help simplify the analysis of CO-OFDM. The performance of developed PLI model in terms of Q factor versus number of subcarriers and Q factor versus fiber length are extensively simulated. Numerical results show that when the OFDM signal passes through fiber link, the noise power expansion occurs. This expansion which is related to the fiber length, number of subcarriers as well as input power, can cause higher degradation in OFDM signal. Through precise analysis, optimum parameters of the transmission system can be chosen. Simulations prove that Q factor of CO-OFDM, whose number of subcarriers is 128, fiber length is 90 km and input power is- 2d Bm, is up to abound 10 d B. The new PLI model has an important reference value on studying how to suppress FWM effects in CO-OFDM system.When the input power is smaller, FWM effect had little impact on the system; when the input is larger, it is necessary to suppress FWM effect in order not to influence the performance of CO-OFDM system.(4) Finally, a few methods to suppressing FWM effect in CO-OFDM system are studied. These methods include dispersion management technology, power allocation method and optical phase conjugation(OPC) technology and so on. Studies haveshown that the contribution of different subcarriers on FWM effect is different. Center subcarriers contribute more than marginal subcarriers on FWM effects. And several major method of suppressing FWM effects is implemented in different CO-OFDM system modules, dramatic results can be achieved if power allocation method combined with other technologies.